Inconsistent watcher.Add errors on different platforms

[tuxmea]

on Linux watcher.Add() returns no Error.
on OS X it returns *os.PathError

[purpleidea]

This is in response to:

err = watcher.Add(current)
if err == syscall.ENOENT

On OSX this apparently doesn't trigger for file not found and generates a different event. Maybe it's just a different error code on OSX? I don't have a mac, so IDK.

[purpleidea]

(that works on GNU/Linux, btw)

[arp242]

For a non-existent file this currently returns:

inotify    syscall.ErrNo    from unix.InotifyAddWatch()
windows    os.SyscallError  from GetFileAttributes(); always explicitly returns os.NewSyscallError()
kqueue     fs.PathError     from os.Lstat; can also fail on unix.Open() and unix.Kevent()
fen        fs.PathError     from os.Stat(); can also fail on os.ReadDir(), AssociatePath()

We can't really change this without breaking compatibility, as people probably have type assertions for this.

Have to look at changing it in v2 to fs.PathError for path errors, and os.SyscallError for errors with the underlying backend.